![]() |
||||||||||||||
Australia: The Land Where Time Began |
||||||||||||||
Acanthostega,
a Stem Tetrapod – Life History Revealed by Synchrotron microtomography
According to Sanchez et al.
the transition from fish to
tetrapod was arguably the most radical shifts in vertebrate history.
For most aspects of these events (Clack, 2012; Niedźwiedzki et
al., 2010; Shubin, Daeschler
& Jenkins, 2006; Friedman,
Coates & Anderson, 2007; Boisvert, Mark-Kurik & Ahlberg, 2008) data have
been rapidly accumulating, though for the earliest tetrapods the life
histories have remained completely unknown, with a major gap in
understanding of these organisms as living animals. The unspoken
assumption that the largest known tetrapod fossils from the
Devonian represent
adult individuals is symptomatic of this problem. In this paper Sanchez
et al. present the first, as
far as they know, life history of a tetrapod dating to the Devonian,
from the deposit of Stensiö Bjerg in East Greenland (Astin et al., 2010;
Blom et al., 2007), which contained evidence of mass-death of
Acanthostega. Sanchez et al. have shown that even the largest
individuals in this deposit were juveniles by the use of propagation
phase-contrast synchrotron microtomography (PPC-SRμCT) (Sanchez et al.,
2012) to visualise the histology of humeri (upper arm bones) in order to
infer their growth histories. It was found that juvenile
Acanthostega
underwent a long early juvenile stage during which their limb bones were
not ossified and individuals grew to almost final size, which was
followed by a late juvenile stage that lasted at least 6 years, in which
they grew slowly and had ossified limbs. Juveniles are suggested by the
late onset of ossification of limbs to have been exclusively aquatic,
and it is suggested by the predominance of juveniles in the sample that
at least at certain times distributions of juveniles and adults were
segregated. The possibility of sexual dimorphism, adaptive strategies or
composition-related size variation is suggested by the absolute size at
which ossification of limbs began differs greatly between individuals.
There has been much speculation on the transition from water to land and
the role the life cycle had in this process. E.g., it has been suggested
that the earliest tetrapods returned to ephemeral pools to reproduce,
and that when the larvae needed to travel over land or through water
that was extremely shallow to relocate from the ponds that were drying
up to water bodies that were more permanent was the selective pressure
to evolve towards terrestriality (Warburton & Denman, 1961). The fossil
record of tetrapods from the Devonian has, however, been dominated by
specimens that were rare and incomplete that are often recovered from
localities that are poorly constrained such as scree slopes (Blom, Clack
& Ahlberg, 2005), has until recently yielded virtually no data on the
life history of these tetrapods.
The deposit in the Britta Dal Formation on Stensiö Bjerg, East Greenland
(Blom, Clack & Ahlberg, 2005), the site of the
Acanthostega mass death,
is the only known tetrapod locality with potential for revealing
information on the life history, which dates to the Devonian. This
locality, which is comprised of a small in situ micaceous silty
sandstone body and scree which is immediately associated (Blom, Clack &
Ahlberg, 2005), has to date yielded more than 200 skeletal elements.
Among the 14 skulls recovered from this site that were associated with
skeletons that were partially articulated, which were complete enough to
measure (Clack, 2002), and there were several more which could be
identified as individuals; Sanchez et
al. estimating that there
were at least 20 animals represented, though there were almost certainly
more present. There were only a few isolated bones that represented
other vertebrates. The individual
Acanthostega that were
recovered from this site appeared to have died together, probably during
a drought that followed a sheet flood event (Astin et al., 2010);
therefore, they represent a single time-point sample from a population
of this stem tetrapod. Sanchez et
al. used the non-destructive imaging technique PPC-SRμCT (Sanchez et
al., 2012), performed at beamline ID19 of the European Synchrotron
Radiation Facility (ESRF) to undertake historical investigations of the
4 humeri that had been collected from the locality (Natural History
Museum of Denmark MGUH 29019, MGUH 29020, NHMD 74756; University Museum
of Zoology Cambridge UMZC T.1295 (Coates, 1996), to recover data that
illuminate the life history of
Acanthostega. All of
these are humeri of
Acanthostega are the only
ones known to date. The other humeri that have been recovered are
isolated bones. These
Acanthostega humeri are
of 2 distinct size categories – large (NHMD 74756, MGUH 29020) and small
(MGUH 29019. UMZC T.1296). Sanchez et
al. found no correlation
between size and the degree to which ossification had progressed, which
is consistent with previous observations (Callier, Clack & Ahlberg,
2009): specimens NHMD 74756 and UMZC T.1295 are weakly ossified while
specimens MGUH 29019 and MGUH 29020 are strongly ossified.
An extensive spongiosa surrounded by a thin compact cortex is exhibited
by all humeri. This arrangement is similar to that of the humerus of the
lobe finned fish
Eusthenopteron (Sanchez,
Tafforeau & Ahlberg, 2014), which a member of the tetrapod stem group
that is less crownward (Coates, Ruta & Friedman, 2008). In the
metaphyseal region, which is close to the articular extremities) there
are remnants of calcified cartilage, which show that the spongiosa
formed by endochondral ossification as occurs in extant tetrapods
(Francillon-Vieillot et al., 1990) and
Eusthenopteron (Sanchez,
Tafforeau & Ahlberg, 2014; Lauren et al., 2007). At the base of the
epiphyses there are tubular structures resembling the marrow processes
in the growth plate of the humerus of
Eusthenopteron (Sanchez,
Tafforeau & Ahlberg, 2014).
There is a dense arrangement of radial vascular canals similar to those
of juvenile
Eusthenopteron in the
midshaft cortex of all
Acanthostega humeri. The
radial canals are connected to a basal mesh of canals that are parallel
to the surface. The largest specimen, MGUH 29020 has radial canals that
vary in diameter between different parts of the area that is being
scanned, which probably reflects local blood supply needs. All appear to
retain areas of primary internal cortical surface, though in 3 of the
humeri there is evidence in the cortex of patchy basal erosion. Between
the endosteal bone and the cortex there are clusters of large aligned
globular cell lacunae that ca be identified as chondrocyte lacunae by
comparing them with juvenile
Eusthenopteron, which
have similar lacunae between the cortical bone and remnants of calcified
cartilage that have not been resorbed (Sanchez, Tafforeau & Ahlberg,
2014). The perichondral surface of the original cartilaginous humerus is
marked by these many alignments of chondrocyte lacunae at midshaft. It
is implied by this that, as limb bone growth originates at the midshaft,
the cartilaginous rod was very large relative to the final size of the
bone that was observed, and that the growth of cortical bone conversely
only made a modest contribution to its final size. I.e., the
Acanthostega individuals
grew to almost full observed size before the humeri began to ossify.
Lines of arrested growth (LAGs) are present in the cortical bone and
this allows the inference of the number of years that were occupied by
the deposition of this tissue, based on the assumption that the deposit
between 2 LAGs represents an annual cycle, which is the case in most
extant tetrapods (Castanet, Francillon-Vieillot & de Ricqlès, 2003;
Padian, 2012). It has been revealed by observations of the 4 humeri that
there are a maximum number of 6 LAGs in MGUH 29020, 4 in NHMD 74756 and
UMZC T.1295, and 3 in MGUH 29019. All observations were made in areas
that were at least partially covered by matrix and therefore are not
likely to have been affected by external erosion. These LAG patterns are
regular and do not show tightening – i.e., there was no growth rate
deceleration – as would be expected at sexual maturity in adult
tetrapods (Padian, 2012; Sanchez, 2008; Castanet et al., 1993). It is
suggested by this that the 4 specimens of
Acanthostega were
juveniles at the time of their death, if it is assumed that their humeri
had begun to ossify prior to the onset of sexual maturity, as they do in
all tetrapods that are known (Fröbisch, 2008; Witzmann, 2006;
Schoch, 2004 and in
Eusthenopteron (Sanchez,
Tafforeau & Ahlberg, 2014). It is suggested by Sanchez et
al., therefore, that the
juvenile stage in
Acanthostega must have
lasted for at least 6 years. They also suggested that it probably lasted
a good deal longer, as the cartilaginous humerus grew to almost full
size prior to the deposition of cortical bone, and therefore the
recording of annual growth increments, even began.
Acanthostega is not the
only member of the tetrapod stem group to display late onset of
ossification. A large spongiosa and a cortex with no internal resorption
is exhibited by juvenile
Eusthenopteron, which
shows that the original cartilaginous rod was about ⅔ of the adult size
of spongiosa, and it is presumed to have formed over several years
(Sanchez, Tafforeau & Ahlberg, 2014). It is difficult to say how this
relates to final adult size in
Acanthostega, though it
is suggested by the slow growth rate of the juvenile
Acanthostega that final
adult size may not have been much greater than the largest individuals
that have been recorded from the mass death deposit.
Sanchez et al. suggest the
complete lack of correlation between size and degree of ossification
could be a reflection of some form of individual variation, such as the
variation of size related to competition that is observed in certain
extant tetrapods (Peacor & Pfister, 2006), adaptive strategies or sexual
dimorphism (Badyaev, 2002). Some individuals, represented by MGUH 29019
and UMZC T.1295, began ossifying their humeri, and presumably approach
sexual maturity, while at a much smaller size than others, which were
represented by MGUH 29020 and NHMD 74756, under these interpretations.
It was not possible because of the small size of the sample to Determine
whether the apparently discrete size classes are a reflection of a real
bimodal size distribution, or whether they are simply the result of a
continuous size variation that was sampled randomly. The construction of
an ontogenetic sequence from smallest to largest humerus was invalidated
categorically, however, by the observed combination of sizes and
ossification states.
New light was shed on several aspects of palaeobiology and life history
of
Acanthostega by the
synchrotron virtual histological data derived from the humeri. As shown
by the LAGs,
Acanthostega had a
prolonged juvenile stage of no less than 6 years, though more probably
at least 10 years, given that it grew to almost full recorded size
before the onset of cortical bone ossification. According to Sanchez et
al. this aligns it with a
range of sarcopterygian fishes and tetrapods that included
Neoceratodus (15-20 years
(Kind, 2002)),
Eusthenopteron (adulthood
at 11 years (Sanchez, Tafforeau & Ahlberg, 2014), Discosauricus
(10 years (Sanchez et al., 2008)) and
Andrias (larval period of
4-5 years and 10 years to adulthood (Sparreboom, 2014)), which suggests
a long juvenile stage could be the primitive condition for tetrapods. It
is implied by the late onset of ossification in
Acanthostega that the
early juvenile stage was aquatic, as a cartilaginous humerus would not
be suitable for locomotion on land; this also agrees with the presence
of aquatic adaptations that include a large caudal fin and a
well-developed gill skeleton in
Acanthostega (Clack,
2012; Ahlberg & Milner, 1994), and contradicts the hypothesis that
juveniles were terrestrial (Warburton & Denman, 1961) at least for this
particular tetrapod.
It is suggested by the fact that all 4 of the humeri appear to be from
juvenile individuals that the mass death assemblage is dominated by, and
could possibly consist exclusively of, juveniles, a set of distinctly
larger individuals is not included in the assemblage. The most fully
ossified humeri of the assemblage were from specimens MGUH 29019 and
MGUH 29020. MGUH 29019, which is the smallest humerus, is associated
with a skull that was 12 cm long; MGUH 29020 is the largest humerus, was
an isolated find from the scree, though it appears to represent one of
the largest individuals in the assemblage (personal observation, J.A.C.
and P.E.A.).
A
context is provided for these observations by the palaeoenvironmental
data from this locality. It forms part of a large ephemeral fluvial
system in what is otherwise an arid topical landscape (Astin et al.,
2010), which extends to the north for more than 200 km from the source
water body that has not been preserved that must have been large and
permanent as it was home to large lobe-finned fishes such as
Eusthenodon and
Holoptychius (Blom et
al., 2007). It appears the
Acanthostega individuals
were flushed out into the fluvial system during a flood event, then an
ensuing drought concentrated them is a shrinking pool that eventually
dried out, which killed them (Astin et al., 2010). It is suggested by
the almost complete absence of other taxa in the death assemblage that
it is not a whole fauna that has been concentrated (as was the case in a
near-contemporary mass death deposit from Canowindra, Australia, rather
it may be a reflection of schooling behaviour in
Acanthostega. Sanchez et
al. concluded, therefore that
Acanthostega had a long
aquatic juvenile stage that was characterised, at least in certain
times, by the formation of schools that included few if any adults.
The type of life history information that is provided by the humeri is
dependent only on the preservation of the actual bone and it potentially
can be matched in a wide range of stem tetrapods, whereas the unique
palaeoenvironmental and population related data that are provided by the
Acanthostega mass death
deposit depend on the context of that particular locality. In principle,
a single limb bone can provide decisive answers to questions concerning
the length of the juvenile stage and at what age the onset of
ossification occurs, which in turn help in constraining the
palaeobiological hypothesis. Sanchez et
al. are undertaking a
systematic PPC-SRμCT survey of the limb histology of stem tetrapods with
this aim.
Acanthostega provides,
for now, a first glimpse of the life history of a tetrapod from the
Devonian.
|
|
|||||||||||||
|
||||||||||||||
Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |